Go to the Plugins dashboard and "Activate" the plugin (for MultiSite users, you can safely "Network Activate" this plugin).

FTP Installation

Download the ZIP file from the WordPress plugin repository.

Unzip the file somewhere on your harddrive.

FTP into your Web server and navigate to the /wp-content/plugins directory.

Upload the collapsible-widget-area folder and all of its contents into your plugins directory.

Go to the Plugins dashboard and "Activate" the plugin (for MultiSite users, you can safely "Network Activate" this plugin).

Must-Use Installation

If you would like to force this plugin to be active (generally only useful for Multi Site installations) without an option to deactivate it, you can upload the contents of the collapsible-widget-area folder to your /wp-content/mu-plugins folder. If that folder does not exist, you can safely create it.